Fever in young infants is a problem commonly encountered by primary care physicians. Because of their developmental immaturity, young infants may not demonstrate clinical signs or behaviors which indicate underlying serious disease. This has led physicians to adopt a wide range of clinical strategies in treating these infants. Currently recommended policies for managing febrile infants vary from extensive diagnostic evaluation, hospitalization and treatment of all infants to the use of clinical judgement to selectively hospitalize and treat those infants who are at risk for serious illness. The purpose of this study is to assess what factors in the management of febrile infants are associated with variations in clinical outcomes. The questions to be addressed include: 1. What is the nature of diagnostic and treatment variability of febrile (> 38.0o C) infants (< 2 months) by office based pediatricians? What are the patient, provider and structural characteristics associated with different management strategies? 2. How effective are current clinical strategies in predicting which febrile infants have serious illness? 3. What are the benefits, harms (morbidity) and costs in febrile infants associated with these varying diagnostic and treatment strategies? 4. Can physicians identify febrile infants for whom the risk of disease is sufficiently low that a "treat-all" policy in unwarranted because the harms (morbidity) and costs of this policy exceeds the benefits and costs of a more selective strategy? We propose to utilize a nationwide group of more than 700 pediatricians in the Pediatric Research in Office Settings (PROS) of the American Academy of Pediatrics to monitor the outcome of a cohort of 6000 febrile infants less than 2 months of age cared for by PROS members. All participating physicians will gather clinical, laboratory, diagnostic and management information on febrile infants in a manner consistent with their usual practice for a two year study period. A follow-up diagnostic assessment will be done at 72-96 hours and if necessary again at 10 days to document the emergence of SBI. Analyses will be conducted to document variation in clinical care, the relative effectiveness of differing clinical strategies in preventing delay in the treatment of SBI, and the effectiveness of current clinical strategies in predicting which infants have serious illness. Recursive partitioning techniques will be used to develop a model for the prediction of the presence or absence of SBI. Different clinical strategies will be compared with respect to their relative benefits and harms. Power analyses have revealed that the sample size is adequate to answer the major questions.